This invention relates to the general area of devices for adjusting the centring of a ring for synchronization of control of pivoting vanes of a turbo-engine on the latter's axis.
In a turbo-engine, it is a known practice to use one or several stages of stator vanes for adjusting the flow and direction of flow of the gases going through the compression section in accordance with the operating conditions of the turbo-engine. These stages of stator vanes comprise a plurality of vanes (called variable pitch vanes) which may pivot about their pins connecting them to the stator in such a way as to alter their pitch angle in accordance with the operating conditions of the turbo-engine.
The known devices for controlling variable pitch vanes usually comprise a ring surrounding a cylindrical shell of the turbo-engine casing and a plurality of levers or connecting rods, each connecting rod having a first end coupled to the control ring by a joint and a second end fitted to the pivot of the respective vane. The synchronized alteration of the angular position of the vanes is thus achieved by rotation of the ring about the turbo-engine axis.
The vane control ring is centred on the turbo-engine axis. In addition, it is arranged coaxially with the shell of the casing of the turbo-engine and generally rests on the latter by means of a plurality of pads, evenly distributed along its full circumference.
In order to improve the precision of stator vane pitch, it is necessary to seek to ensure excellent concentricity of the control ring with the casing shell. Perfect concentricity can be obtained by maintaining a very small clearance between the pads and the casing shell. To obtain a very small clearance, the devices used for fitting the pads to the control ring must therefore make it possible for the pads to be adjusted in a radial direction.
Devices for adjusting control ring concentricity generally use pads fitted to threaded rods which are screwed into the control ring. Also, from document U.S. Pat. No. 5,387,080, pads are known, the radial position of which is adjusted by means of a double screw which is fitted to the ring and the two portions of which have slightly different helix angles.
These devices for adjusting the pads in the control ring are not capable of providing perfect immobilization of the pad adjustment threaded rod during all the operating phases of the turbo-engine. In fact, when some pads are no longer in contact with the casing shell, wear of the adjustment rod threads is caused by vibration, which leads to loss of adjustment of the pads and consequently vibration of the control ring, this vibration being particularly detrimental in terms of vane pitch precision.